Adding fall blooming plants may help both managed and wild bees in cities

UNIVERSITY PARK, Pa. — There are nearly 4,000 native bee species in the United States, contributing to pollination in agricultural, urban and natural landscapes. Honey bees, however, are not native to the U.S., which has led to concerns that managed honey bee populations might negatively impact wild bees via competition for resources and sharing of pathogens, according to researchers at Penn State. However, the team recently found that of the 33 genera of native bees studied, only a small number seemed to be negatively affected by the presence of honey bees.

The study, the largest of its kind, is available online now and will appear in the November issue of Science of The Total Environment.

The findings will help identify the groups of bees that may be most at-risk and aid in creating conservation strategies, according to Gabriela Quinlan, lead author on the study and a U.S. National Science Foundation (NSF) postdoctoral research fellow in Penn State’s Center for Pollinator Research and the College of Agricultural Sciences.

The researchers found that the presence of managed honey bee apiaries and/or urban land correlated with reduced numbers of bees from six of 33 wild bee genera, suggesting that one these factors negatively impact these bees’ ability to maintain their populations. Since the honey bee apiaries in the study were located in greater densities in urban areas, the researchers sought to disentangle the relative role of apiary density versus developed land. 

The six bee genera showed different responses to these two factors, with apiary density having the greatest negative impact on the long-horned bees in the genus Svastra, while abundance of Florilegus, which is also a long-horned bee, was associated most negatively with urbanization. The others included green bees in the genera Agapostemon and Augochlora, sweat bees in the genus Lasioglossum and long-horned bees in the genus Melissodes.

After analyzing the data, the researchers found the genera most affected by apiaries and land development tend to nest in the ground, forage later in the season and, in some cases, specialize on specific types of seasonal flowering plants. 

“Urban environments can be especially challenging for ground-nesting bees because the hardscaping — asphalt and concrete — can make it difficult to find bare ground for nesting,” Quinlan said. “Small bees can’t fly very far, so they may have trouble finding nesting and flowering resources within their flight range. Moreover, specialist bees require very specific flowers, which may be difficult to find in urban settings.”

Quinlan explained that some wild bees also forage later in the season, which happens to be when honey bee colonies are at their peak size and competing for limited floral resources. 

“Planting late-season forage and leaving bare ground as nesting resources in urban gardens could help alleviate some of the stressors that developed land and urban beekeeping can put on these bee groups,” Quinlan said.

For their study, the researchers obtained the locations of nearly 4,000 registered apiaries across the state of Maryland and used this information to build a state map of apiary density. They then compared this with data on wild bee populations from within the state’s boundaries collected by the U.S. Geological Survey’s Bee Monitoring and Identification Lab.

In recent decades, the researchers said, there has been wide recognition that pollinators are experiencing declines associated with pesticides, pathogens, poor nutrition and climate change. There also has been increased attention paid to how honey bees may affect wild, native bees by transmitting pathogens and exacerbating competition for resources.

But despite these strong concerns, evidence of the effects honey bees have on wild bee abundance was lacking in the literature. 

“There is very little data on population sizes of most wild bee species across the U.S., so we did not have a good understanding of the factors that influence the abundance of different wild bee species,” said Christina Grozinger, Publius Vergilius Maro Professor of Entomology, director of the Center for Pollinator Research and co-author on the study. “Here, we explicitly tested the question of whether the presence of managed honey bee populations was linked with reductions in wild bee abundance.”

The study provided additional insights into how different bee genera, with different natural histories, are influenced by environmental factors, Quinlan noted.

For example, they also found that apiary density negatively affected the abundance of Triepeolus — a cuckoo bee — which parasitizes the nests of long-horned bees like Svastra. In this case, it may be the negative impact of apiary density on Svastra abundance that causes a negative effect on Triepeolus abundance. Grozinger noted that this highlights the interconnectedness of these different species, and the importance of thinking about bees as communities, not as individual species.

The researchers said that in the future, additional studies could be done in other areas of the U.S. to further tease out genera- and species-specific wild bee responses to honey bees. The new INSECT NET graduate training program at Penn State is aiming to develop nonlethal, automated monitoring systems for bees and other insects that will help with these studies.

In the meantime, Grozinger said one of the best ways people can help both wild and managed bees is to add more flowering plants to the landscape, including city streets and backyards. 

“Native flowering trees and shrubs in particular provide a bonanza of resources for bees,” she said. “Beekeepers can help their bees become more resilient to climate variation by managing pests and diseases within their colonies, and this can also reduce the potential for honey bees to spread diseases to wild bees.”  

People can learn more about the resources available to bees in their area, and the risks posed by pesticide use and weather conditions, by using Penn State’s Beescape decision-support tool. 

Jeffrey Doser, of Michigan State University, and Melanie Kammerer, senior geospatial data scientist at EcoData Technology and Penn State doctoral alumna in ecology, also co-authored this study.

The U.S. Department of Agriculture’s National Institute of Food and Agriculture and Hatch Appropriations and the NSF Postdoctoral Research Fellowship in Biology Program helped support this research. For more information about research on managed and wild bees and what you can do to help bee populations, visit the Penn State Center for Pollinator Research website.